This invention relates to insulation requirements for power transformer windings wherein troublesome high frequency voltage oscillations can be induced by occurrence of lightning impulses at the transformer terminals. The magnitude of the internal voltage oscillations can be controlled to some extent by the application of various electrostatic shielding techniques. For transformers employing concentric clyindrical windings, the shielding frequently takes the form of an outer enclosing cylinder which is electrically connected to the line terminal of the winding. This cylinder is made up of a plurality of metal foil strips which are heavily insulated from the winding and from surrounding structural parts at ground potential. The electrostatic shield can not consist of a continuous metal enclosure since the presence of strong electromagnetic fields would induce currents in the shield, producing losses and heating.
Since the above described form of electrostatic shield completely encloses the winding, it effectively eliminates most of the dielectric stress to ground from the winding turns and concentrates the stress at the surface of the shield, and, in particular, at the ends of the shield where the spacing to ground is a minimum. To accomodate this stress concentration, the ends of the shield normally are formed with a large radius of curvature and are heavily insulated. To avoid any dielectric weakness, the shield insulation must completely enclose the total shield electrode and is carried continuously over both the inside and outside surfaces of the shield. The presence of thick insulation between the inner surface of the shield and the outer layer of the winding turns reduces the shield to winding capacitance and decreases the effectiveness of the shield in controlling intrawinding voltage oscillations.
The purpose of this invention is to provide an improved shield for power transformers to substantially increase the shield to winding capacitance and reduce the magnitude of transient voltage oscillations with the transformer winding produced by external lightning surges.